Method of thread rolling

ABSTRACT

A method of forming threads on ultra high strength metal alloys by preforming the threads in the annealed condition while allowing a segment of material below the pitch diameter to be compression cold rolled after heat treatment especially at the root radius. Thread forming at Rockwell C50 to C65 hardness greatly reduces the life of thread rolls. This method entirely overcomes this failure while maintaining the required tensile fatigue life of the bolt threads.

United States Patent Inventor Emrlc W. Bergere 2324 Nottingham Ave., Los Angeles, Calif. 90027 Appl. No. 773,192

Filed Nov. 4, 1968 Patented July 6, 1971 METHOD OF THREAD ROLLING 2 Claims, 3 Drawing Figs.

U.S. Cl. 10/152, 10/10, 10/27, 10/86, 72/103 Int. Cl B2111 3/02, B2lh 3/04, B2111 3/08, 823g 7/00, 823g 9/00 Field of Search lO/lO, 27, 27.l, 86, 141, 1, 152, 152 T; 85/46, 48; 151/22;

Primary Examiner-Charles W. Lanham Assistant Examiner-E. M. Combs Attorney-Howard l. Podell ABSTRACT: A method of forming threads on ultra high strength metal alloys by preforming the threads in the annealed condition while allowing a segment of material below the pitch diameter to be compression cold rolled after heat treatment especially at the root radius. Thread forming at Rockwell CS0 to C65 hardness greatly reduces the life of thread rolls. This method entirely overcomes this failure while maintaining the required tensile fatigue life of the bolt threads.

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, METHOD OF THREAD ROLLING It is required that threads be rolled after heat treating to maintain a specified minimum tensile fatigue life in bolts made of alloys used at high tensile preloads, especially in diameters over five sixteenths inch. Economical life of thread rolling dies is maintained when these tools, with maximum workable hardness, can have a Rc20 point hardness difference above the blanks rolled. As the dies are made to Rc65 maximum, parts at Rc55 leave only a point margin and this reduces the tooling life four times. Maraging steels at Rc60, in diameters over five-eighths inch are very difficult to cold roll. Warm rolling is a method used to overcome this difficulty but with a reduction of fatigue life and at higher production cost.

This invention overcomes the problems of thread rolling tough and rapidly cold-working materials by reducing the area of cold rolling by preforming before heat treatment using a new thread form which has a completed standard thread above the pitch diameter. The thread form, below the pitch diameter, has a calculated segment at the flanks and root to allow compressive thread rolling after heat treatment and so hold the higher fatigue life as specified. Roll or flat dies will be equally satisfactory for this improved method.

A better understanding of the invention will be had by referring to one embodiment thereof as illustrated in the accompanying drawings, in which:

FIG. 1 shows in cross section a series of threads rolled on the bolt in the annealed material condition.

FIG. 2 illustrates by the cross section the threads rolled in the heat treated material condition.

FIG. 3 is an enlarged cross section of the root section of one thread.

The enumerated drawings show the screw thread-shape and the outlines of the forming rolls or dies which produce bolts in the well-known manner. The machine perfonning these operations is familiar to those engaged in this art and descriptive reference is therefore unnecessary.

The drawing shows in FIG. 1 a bolt 10 and pitch diameter 11 above which a transition line 12 marks the break in the thread flank angle between the lower section with slope 17 and the upper section with slope 18. Points 120 and 12b on line 12 mark the angular break of the thread contour. The upper triangular sectional area, bounded by the thread crest 13 and with its base along line 12, is formed by roller 15 or chase cut to the finished contour and dimensionsin the annealed condition of the bolt material. The lower area, below line 12 is formed with flanks 17 and root radius 14a along the minor diameter line 14 to allow material for subsequent rolling after heat treatment.

Roller 22 in FIG. 2 shows that cold forming contact is made only below line 12. Roller faces 25 clear the flanks 18 to points 12a and the thread crest is clear by the roller depth at 23. Flanks 21 and root radius 24a along the minor diameter 24 are the cold worked areas and performed after heat treatment of the bolt to improve the fatigue life in the tension-tension type testing especially specified in aerospace requirements. The finish roll does not touch the upper triangular thread section above line 12.

The enlarged section shown by FIG. 3 and encircled in FIG. 1 by 3-3 details the cold worked condition obtained by roller 22. Line 12, the flank angle convergence points 12a and 12b, is defined as 0.06 p minimum (thread pitch p) above the pitch diameter and varying with tolerance allowances. In the 70 first operation thread form the segment for the cold rolling area is outlined-starting from the left, by point'lZa along line 12, flank 17 and root radius 140, flank I7 and to point 12b. After cold working, the standard 60 final operation thread will be as outlined by points 12a and 12b, flanks 21 and root radius 240. The compression area of the minor diameter, between line 14 and the final contour is equal to 0.05 p minimum, varying with tolerance allowances. The solid line of the bolt 10 denotes the thread formed in the annealed metal condition and the dashed line outlines the volume which is cold worked after heat treatment.

In this invention the first operation contour can, by the required production choice, be single pointed, die cut, chased, trirolled, duo-rolled or flat die rolled without detrimental loss of fatigue life which is determined by subsequent compression rolling of the cold worked areas after heat treatment.

This invention is not to be limited to the embodiments shown in the drawings and herein described, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.

What I claim then is: l. A method for producing a metal fastener with improved holding strength under reversal tension fatigue conditions, having a screw thread for threaded engagement with a mating member, said screw thread being selectively work hardened in manufacture by thread rolling means so as to compress the metal to a greater extent in the dedendum sector of the screw thread, said method comprising the steps of forming an initial screw thread on a metal blank to the dimensions of the desired final thread form in the addendum sector of the screw thread, said initial screw thread being dimensionally oversize the desired final thread form in the dedendum sector of the screw thread, thread rolling said initial screw thread to the final thread form, utilizing thread rolling dies shaped to form the desired final thread on the screw in the dedendum sector of the screw thread form, said thread rolling dies being dimensionally relieved in the addendum forming sector, so as to apply a greater amount of thread rolling pressure to the dedendum sector of the screw thread than to the addendum sector of the screw thread where the addendum sector of the screw thread is that portion of the screw thread extending from the crests of the screw thread to the proximity of the pitch line and the dedendum sector is the remainder of the screw thread extending to the root of the thread resulting in the compression of the grain of the metal in the dedendum sector of the screw threads and beneath the roots of the screw threads thereby improving the fatigue life of the screw in tension without creating excessive work hardening strains in the addendum sector of the screw threads.

2. The method described in claim 1 wherein the blank is heat treated after being formed to the initial screw thread, so as to relieve the work hardening strains of forming the initial thread, and subsequently thread rolled to the final thread form after said heat treating so as to work harden the metal of the screw in the dedendum sector of the screw threads in the heat treated condition leaving the metal of the screw thread in the addendum sector of the screw threads in the strain relieved condition. 

1. A method for producing a metal fastener with improved holding strength under reversal tension fatigue conditions, having a screw thread for threaded engagement with a mating member, said screw thread being selectively work hardened in manufacture by thread rolling means so as to compress the metal to a greater extent in the dedendum sector of the screw thread, said method comprising the steps of forming an initial screw thread on a metal blank to the dimensions of the desired final thread form in the addendum sector of the screw thread, said initial screw thread being dimensionally oversize the desired final thread form in the dedendum sector of the screw thread, thread rolling said initial screw thread to the final thread form, utilizing thread rolling dies shaped to form the desired final thread on the screw in the dedendum sector of the screw thread form, said thread rolling dies being dimensionally relieved in the addendum forming sector, so as to apply a greater amount of thread rolling pressure to the dedendum sector of the screw thread than to the addendum sector of the screw thread where the addendum sector of the screw thread is that portion of the screw thread extending from the crests of the screw thread to the proximity of the pitch line and the dedendum sector is the remainder of the screw thread extending to the root of the thread resulting in the compression of the grain of the metal in the dedendum sector of the screw threads and beneath the roots of the screw threads thereby improving the fatigue life of the screw in tension without creating excessive work hardening strains in the addendum sector of the screw threads.
 2. The method described in claim 1 wherein the blank is heat treated after being formed to the initial screw thread, so as to relieve the work hardening strains of forming the initial thread, and subsequently thread rolled to the final thread form after said heat treating so as to work harden the metal of the screw in the dedendum sector of the screw threads in the heat treated condition leaving the metal of the screw thread in the addendum sector of the screw threads in the strain relieved condition. 